The invention relates to a device and a method for the physical separation of solid matter from suspensions using the flotation method in an essentially cylindrical separating tank. Devices of this type are, for example, known from EP-A-0 442 463 or WO 95/23027.
Based on these previously known devices or methods, the object of the present invention is to further improve the degree of separation in the separation of solid matter of generic devices and methods.
A rotationally symmetrical flocculation reactor, closed at the top and which preferably expands downward in its diameter, is arranged centrally in the middle in the cylindrical separating tank.
The suspension inlet does not take place concentrically in the flocculation reactor but preferably via a vertical inlet pipe, the eccentrical supply and the inlet pipe, preferably open on one end, resulting in the formation of a gentle, downward-directed and decelerating circulation flow which decisively promotes the flocculation process required for the separation of solid matter by flotation. An overflow edge, situated on the periphery and enabling the suspension to flow out of the flocculation reactor, is found at the lower end of the rotationally symmetrical flocculation reactor.
The liquid/suspension flowing out of the flocculation reactor flows into distribution cells situated on the outer periphery of the flocculation reactor, said distribution cells having a geometrical form such that a tangentially directed flow into the separating tank results at the end of the distribution cells, i.e. an inlet angle that also has, in addition to the radially directed flow component, a tangential flow component.
A pressure-release inlet is arranged in the flocculation/flow reactor in which the release pressure of a liquid oversaturated with gas can be reduced via an annular aperture. The bubbles resulting from the excess gas form a carpet of bubbles which flows radially outward from the pressure-release inlet in an almost horizontal direction, comes to lie beneath the gentle, downward-directed decelerating circulation suspension flow resulting in the flocculation/flow reactor and/or mixes with it. As a result of the adjustable annular aperture of the pressure-release inlet, an operation at various release pressures is possible which enables an optimum control of the bubble sizes that is dependent on the absolute level of the release pressure.
The solid floccules resulting in the circular flow of the flocculation/flow reactor are thus optimally underlaid or mixed with a bubble carpet and tangentially introduced into the separating tank via the distribution cells disposed on the outer periphery of the flocculation reactor.
In a preferred embodiment, the distribution cells are formed in such a way that the tangential components of the direction of flow are larger than the radial components. Even more preferable, the ratio of the tangential components to the radial components is 1:1 to 50:1, especially preferred 2:1 to 5:1.
In order to use flotation more successfully as chemical and physical separating methods, an optimal formation of solid floccules/solid bonds is an essential prerequisite for obtaining good separation results, whereby the use of chemical flocculation adjuvants (e.g. polymers) are prior art nowadays, however, the costs of these flocculation adjuvants represent a considerable operating cost factor.
The use of flocculation adjuvants or solid flocculation can be considerably optimized thereby that the flocculation itself takes place in its own reaction chamber at defined, non-turbulent flow conditions while maintaining specific, even the shortest reaction times. This was not taken into consideration in the previous prior art.
The present invention optimally meets these requirements for an improved flocculation and enables the reduced use of flocculation adjuvants with the flocculation/flow reactor in which a gentle downward-directed circulation flow is produced by the eccentric inflow.
In the previous prior art, the mixing of the bubbles produced by oversaturation mostly takes place in pipes or narrow flow chambers at high flow velocities of about 1.6 to 3 m/s, in addition, the paths from the bubble formation until mixing and to the start of the effective flotation are long (3 to 10 m). Both factors result in disadvantages; high flow velocities and long paths destroy formed floccules or inhibit/prevent flocculation, the smallest bubbles produced by oversaturation coalesce to form larger bubbles which are less effective for flotation.
On the other hand, in the present invention, the bubbles are gently mixed in without affecting the floccules in the flocculation/flow reactor at reduced velocity, short paths of preferably 0.5 to 1 m up to the separating tank in which the flotation takes place, prevent coalescence of the bubbles and enable improved flotation with the smallest bubbles.
A further advantage of the invention lies therein that the flow of the mixture consisting of suspension and air bubbles that is usually directed purely radially outward must travel a longer path to the outlet openings in the area of the tank wall due to the tangential flow component than in conventional separating methods with purely radial flow, so that the separation of solid matter is carried out in a more effective manner due to the longer paths or the longer stay in the separating tank. In addition, the flow-off is also calmer which also results in an improved separation. Thus, the device of the invention can be configured more compact vis-a-vis previously known designs.
A preferred further embodiment of the invention provides that the pressure-release inlet comprises an axially movable, preferably conical adjusting piston, a more or less narrower annular aperture being formed by its axial adjustment, the liquid mixed with the released gas passing through said annular aperture under the sudden fall in pressure. By adjusting the aperture, depending on the desired flow throughput, the fall in pressure can be set in such a way that a suitable bubble size results which is advantageously between 10 xcexcm and 1500 xcexcm.
An advantageous further embodiment of the invention provides that deflecting devices be provided above the pressure-release inlet which serve to bundle the flow of the bubble-added liquid in horizontal/radial direction, so that a two-layer flow is formed, the bubble-added liquid running at the bottom of the tank and the suspension flow above it.
As soon as the liquid freed of solid material reaches the area of the outer wall of the separating tank, it is removed from the separating tank via specially designed outlet openings and conveyed to a liquid outlet via collecting pipes. Preferably, the outlet openings are configured in a slot-shaped manner, these slots being located over the entire periphery of the wall of the tank above the floor. These outlet openings are brought together in groups by suitable channels or lines and communicate with a number (preferably 4) of outlets distributed along the periphery of the tank.
An arrangement to be provided alternatively or in addition for draining the clarified liquid lies therein that at least one surrounding discharge pipe is situated on the outer wall of the separating tank above the base of the tank and provided with a plurality of outlet ports. Preferably, these outlet ports are situated in the lower area of the discharge pipes, so that the inlet into the discharge pipe takes place from the bottom, in order to effectively prevent sinking substances from blocking the openings. This discharge pipe communicates via a connecting line with an outlet. Alternatively, it is also possible to divide this discharge pipe into several, preferably 3-4 discharge pipe sections, which together produce a complete ring and each communicate with their own outlets. Especially simple from a technical point of view, the discharge pipes may be realized if they consist of straight pipe pieces connected to one another and, in this way, form a type of polygonal discharge pipe.
According to a further advantageous embodiment of the invention, a rotating sludge removal device is provided above the separating tank which preferably comprises at least two sludge paddles which both turn about their own common axis of rotation inclined slightly to the horizontal and about the vertical axis of the separating tank.
An especially preferred further embodiment of the invention provides that the direction of rotation of the sludge-removal device travels opposite to the tangential component of the flow of the suspension, as a result of which the relative velocity is increased between the sludge paddles and the sludge flow in the upper area of the filled tank which leads to an improved and more effective discharge, in particular of the solid fibers contained in the suspension.
An especially effective removal of the sludge can be obtained with a further embodiment according to the invention in which several scooping devices are provided which are distributed about the periphery of a rotating shaft and which convey the sludge in a discharge outlet extending parallel to the shaft, each scooping device consisting of a bow-like blade and a sludge paddle. The sludge is cut into by means of the bow blade, preferably configured rectangular, and the removal of the loosened sludge thus facilitated by the following sludge paddle. This further embodiment has special advantages with heavily packed, encrusted or otherwise hardened sludge formations.
In a simple embodiment, the base of the separating tank can be configured flat. A preferred alternative embodiment of the invention provides that the base is configured in an annular channel-like manner and comprises at least one sediment outlet, as a result of which heavy parts deposited during the separating process can be effectively removed.
The aforementioned embodiments of the invention produce a separation of the suspension on the flotation principle. Alternatively, it is also possible to carry out a separation on the sedimentation principle, whereby especially the introduction of a liquid mixed with gas is omitted, i.e. only a suspension inlet is provided for introducing the suspension to be freed from solid matter, it being possible to convey this suspension through the separating tank in the same way as in the embodiments described above through the eccentric arrangement of the suspension inlet with a tangential velocity component.